Download a manual from Browning or take it to a gunsmith
Yes, Browning has made them.
The Browning Auto-5 came in 12-, 16- and 20-gauge varieties.
The last 2 letters in your serial number NZ indicate that your Browning 325 in 20 gauge was made for browning in the year 1991.
Citori, 20 gauge, superlight and upland models
2003; you will have to call Browning for model
Browning has sn data on their website.